1. Field of the invention
The present invention relates to a coating type medium for high density magnetic recording such, for example, as oxide type platelike magnetic powder used as a material of high density magnetic tapes and high density floppy disks, and more particularly to a manufacturing method thereof and a magnetic recording medium which uses the magnetic powder.
2. Description of the Prior Art
Coating type magnetic recording medium is extensively used currently in the form of tapes for audio apparatuses, tapes for video tape recorders, floppy disks, and the like. In the past, .gamma.-Fe.sub.2 O.sub.3 (gamma iron oxide) acicular magnetic powder or Co coated .gamma.-Fe.sub.2 O.sub.3 acicular magnetic powder have been used for the medium of this coating type. Because the magnetic spin axes of these acicular magnetic powders point to the acicular direction thereof, the acicular magnetic powder is suitable as the magnetic powder used for the coating type medium of a longitudinal magnetic recording system.
In recent years, there has been an increasing demand among coating type media for higher magnetic recording density. In order to increase the magnetic recording density, magnetic powder is required to be composed of super minute particles from the aspect of the medium noise. However, as the size of particles of the abovementioned acicular magnetic powder is made smaller, it is impossible to manufacture magnetic powder of very fine particle size because the acicular magnetic powder has a property which causes loss of magnetism (super paramagnetism).
On the other hand, a totally different magnetic oxides from the abovementioned magnetic powder in chemical composition is a hexagonal ferrite magnetic powder, which has a magnetoplumbite type crystal structure. This powder is a magnetic powder in the hexagonal platelike form. A typical example is a hexagonal platelike magnetic powder of barium ferrite (BaFe.sub.12 O.sub.19) type. Because of its very large uniaxial magnetic anisotropy, this powder is characterized by the fact that it does not lose its magnetism even if the powder is made into super minute particles. Therefore, this magnetic powder arrests attention as a magnetic powder that can be used for a high density magnetic recording medium.
Further, as compared with the abovementioned longitudinal magnetic recording system, a perpendicular magnetic recording system is given attention as a system which makes possible far higher density recording. Because it is difficult to manufacture a coating film on which conventional acicular magnetic powder are perpendicularly arranged in a row on the film surface, it is difficult to use the abovementioned acicular powder in view of its shape, as a magnetic powder for magnetic recording medium corresponding to the perpendicular magnetic recording system. The abovementioned hexagonal platelike magnetic powder also draws attention as magnetic powder for the coating type medium correspondint to the perpendicular magnetic recording system. This has arisen from the magnetic characteristics of this powder. That is, this powder comes in hexagonal platelike form with grown C plane ((00l)-plane) and has a construction wherein the magnetic spin axis is caused to point in the perpendicular direction (c axis) to this plate surface (for example, Osamu Kubo, Applied Physics (Japan), Volume 55, No.2 (1986), page 135).
As stated above, the magnetic spin axis points perpendicularly to the plate surface of the platelike particle has arisen from the fact that the magnetic spin axis has magnetoplumbite type crystal structure. Crystal structure units of magnetoplumbite type oxide of iron represented by barium ferrite are composed of two S blocks having the same structure as the spinel type crystal structure comprising iron and oxygen and one R block comprising barium-iron-oxygen (H. Kojima: Ferromagnetic Materials, ed, E. P. Wohlfarth, North-Holland Publishing Company, Amsterdam, 1982, p. 318 to 323).
A schematic diagram showing this crystal structure is shown in FIG. 1. Sandwiching the R block, the S blocks are disposed in vertical direction to form the stacked S-R-S construction. In addition, the stacking direction of this S-R-S construction coincide with the C axis direction of the the magnetoplumbite construction. Viewing each of the S blocks of the spinel type crystal structure, the direction (111) coincides with the stacking direction of the S-R-S construction. Because the iron ion disposed with five oxygen existing in the R block at the center of the unit of this magnetoplumbite construction acts so as to orient the magnetic spin axis of the vertical S blocks into the same direction, the magnetic spin axis will eventually exist to face the S-R-S stacked direction (direction of c axis), that is, to face a direction perpendicular to the plate surface of the hexagonal platelike particles (Soshin Chikazumi: Physics of Ferromagnetism, vol. I, (Syokabo, Tokyo, 1978) p. 228 to 230, [in Japanese], and N. Fuchikami, J. Phys. Soc. Japan, 20, 760 (1965)).
Therefore, when manufacturing a coating type medium, by providing on the base film a coating film of a structure in which hexagonal platelike particles are arranged in close rows, the magnetic spin used for magnetic recording will face toward the perpendicular direction with respect to the surface of the medium, so that a coating type media of perpendicular magnetic recording system can be realized. In other words, this medium shows particles in the form of hexagonal platelike configuration and can be achieved only by the existence of hexagonal ferrite type oxides having magnetoplumbite type crystal structure. FIG. 2 shows a model of the coating type perpendicular magnetic recording medium which uses the abovementioned hexagonal platelike magnetic powder of hexagonal ferrite.
In order to manufacture a recording medium which uses the perpendicular magnetic recording system such as above, it is necessary to orient the abovementioned hexagonal platelike magnetic powder of hexagonal ferrite so that the platelike surface (the c plane of hexagonal ferrite in terms of crystallography) thereof is parallel to the running surface of the magnetic head of the medium, and to coat the magnetic powder on a substrate such as a base film. Because the magnetic powder is oriented and coated in the above manner, various coating methods are designed. Generally, as the platelike ratio of the magnetic powder particles, that is, the diameter/thickness ratio, is greater, the orientation becomes easier.
Of the magnetic characteristics of magnetic powder, the saturation magnetization .sigma.s and the temperature change .DELTA.Hc/.DELTA.T of coercive force Hc in particular largely influence the characteristics of the coating type magnetic recording medium which uses such magnetic powder. As the value .sigma.s of the magnetic powder increases, the signal output of the medium increases. Further, because the coercive force Hc of the magnetic powder influences the stability and writable performance of the recording signal of such medium, as the value .DELTA.Hc/.DELTA.T becomes smaller, it becomes more possible to manufacture an excellent medium.
The .gamma.-Fe.sub.2 O.sub.3 acicular iron oxide of the spinel type crystal structure used conventionally in the longitudinal magnetic recording has a large saturation magnetization value of .sigma.s=70 to 80 emu/g and a temperature change .DELTA.Hc/.DELTA.T of coercive force which has a small negative value of about -1 Oe/deg in the vicinity of the room temperature. However, the hexagonal ferrite magnetic powder of magnetoplumbite type stated above has a relatively small value of .sigma.s and a large positive value of .DELTA.Hc/.DELTA.T in the vicinity of the room temperature. For example, in the case of barium ferrite hexagonal platelike magnetic powder, which is a typical hexagonal ferrite magnetic powder, the saturation magnetization value is .sigma.s=57 emu/g and the temperature change of coercive force is .DELTA.Hc/.DELTA.T.perspectiveto.+3 to +6 Oe/deg. (For example, T. Fukaya, T. Oguchi, H. Takeuchi, S. Hideyama and H. Yokoyama, Jornal of Japan Society of Applied Magnetics, vol. 10, p. 81 (1986) [in Japanese])
As a manufacturing method of these hexagonal ferrite magnetic powders, methods such as the hydrothermal method and glass crystallization method have been established. (For hydrothermal method, refer to M. Kiyama, T. Takada, N. Nagai and N. Horiishi, "Advances in Ceramics, vol. 15, Fourth International Conference on Ferrite, Part 1" (The American Ceramic Society), p. 51 (1986), and for glass crystallization, refer to O. Kubo, T. Ido, and H. Yokoyama, IEEE Transactions on Magnetics, vol. MAG-18, p. 1122 (1982), for example).
These methods are suitable for manufacturing single crystal fine particle of balanced stable phase having stoichiometric composition.
In addition to the hydrothermal method and glass crystallization method, coprecipitation and tempering method are also available as another manufacturing method of conventional hexagonal ferrite (For example, K. Haneda, C. Miyama and H. Kojima, Journal of The American Ceramics society, vol. 57, p. 354 (1974).).
This is a method wherein coprecipitated superfine particle raw material powder which includes Fe and M (M=Ba, Sr, Pb) with mol ratio of 12/1 as [Fe]/[M] is tempered in the air to synthesize hexagonal ferrite magnetic powder. With this method however, because the atmosphere used for tempering is the air, that is, the oxidizing atmosphere, hexagonal ferrite phase of magnetoplumbite structure is not formed directly from the raw material powder, but the non-magnetic .alpha.-Fe.sub.2 O.sub.3 (hematite) phase is formed as an intermediate quasi-stable phase during reaction process. Hexagonal ferrite phase is formed after the intermediate quasi-stable phase is changed again. Further, because it requires a high temperature of over 850.degree. C. to change from the .alpha.-Fe.sub.2 O.sub.3 phase to the hexagonal ferrite phase, hexagonal ferrite magnetic powder cannot be obtained unless the raw material powder is fired at a temperature above 850.degree. C. Because the high temperature tempering is necessary, the coprecipitation and tempering method has such defects that (i) large particles that have grown abnormally are liable to be mixed in the magnetic powder to be manufactured and (ii) a crushing process is required in the manufacturing process of magnetic powder because the powder is liable to cause sintering.
Furthermore, because the raw materials are fired in the oxidizing atmosphere with this coprecipitation and tempering method, when coprecipitated powder which contain more Fe ions than the stoichiometric mol ratio (12:1) as compared to M ions are fired, iron of excessive quantity which is over 12 times the mole number of M becomes the non-magnetic .alpha.-Fe.sub.2 O.sub.3 phase. Thus the lowered magnetic characteristics of the synthesized magnetic powder (particularly of the saturation magnetization) result. In other words, only the hexagonal ferrite magnetic powder of stoichiometric composition can be obtained even if the coprecipitation and tempering method is used.
That is to say, in the conventional methods, only the magnetoplumbite type hexagonal ferrite (MFe.sub.12 O.sub.19 (M=Ba, Sr, Pb)) magnetic powder of materially stoichiometric composition can be manufactured.